1. Field of the Invention
The present invention relates generally to vibrating beams, including piezoelectric or silicon beams that may be piezoelectrically, electromagnetically, electrostatically or thermally driven, and particularly to vibrating beams that are utilized as force sensors, for example, accelerometers. In particular, the present invention relates to a method and apparatus for reducing the reaction forces transferred to the beam supporting structure to thereby improve the mechanical Q of the vibratory system.
2. Description of the Prior Art
Various transducers, including accelerometers, utilize one or more vibrating beams that vibrate laterally or in various other modes. The resonant frequency of such a beam is raised when the beam is subject to tension and lowered when subjected to compression. The transducer is designed so that the physical quality to be measured results in tension or compression being applied to the vibrating beam or beams so that the frequency of vibration of the beam or beams is a measure of the amplitude of the quantity being measured.
The performance of a vibrating beam is degraded if there is a transfer of energy to other structures, for example the beam supporting structure through reaction forces at the ends of the beam. Such mechanical coupling between the beam and supporting structure can lower the Q of the beam and cause undesirable frequency shifts.
One prior art method used to reduce the problems of energy transfer is the utilization of multiple beams vibrating out of phase to cancel reaction forces as is done in the case of a double-ended tuning fork. The double-ended tuning fork utilizes two beams located side-by-side that vibrate in opposite directions to cancel the reaction forces. Examples of multiple beam resonators utilized to cancel reaction forces are disclosed in U.S. Pat. Nos. 4,215,570; 4,372,173; 4,415,827 and 4,901,586.
Another approach used to reduce the transfer of energy from the beam to the mounting structure is to employ vibration isolators between the ends of the beams and the supporting structure. Such isolators usually have an isolation mass at each end of the vibrating beam and a resilient member between each isolation mass and the supporting structure. The resilient members permit the beam and the isolator masses to move relative to the supporting structure in order to reduce the amount of energy transferred from the vibrating mass to the supporting structure.
The isolation systems are most effective when the isolator masses are large and the isolation springs are compliant. Such large isolator masses and compliance springs result in a low resonant frequency for the isolation system which is undesirable, particularly in accelerometer applications. In addition, isolation systems attenuate the reaction forces applied to the supporting structure, but cannot completely eliminate them.